Process and apparatus for processing a nitride structure without silica deposition

ABSTRACT

Techniques are provided to remove the growth of colloidal silica deposits on surfaces of high aspect ratio structures during silicon nitride etch steps. A high selectivity overetch step is used to remove the deposited colloidal silica. The disclosed techniques include the use of phosphoric acid to remove silicon nitride from structures having silicon nitride formed in narrow gap or trench structures having high aspect ratios in which formation of colloidal silica deposits on a surface of the narrow gap or trench through a hydrolysis reaction occurs. A second etch step is used in which the hydrolysis reaction which formed the colloidal silica deposits is reversible, and with the now lower concentration of silica in the nearby phosphoric acid due to the depletion of the silicon nitride, the equilibrium drives the reaction in the reverse direction, dissolving the deposited silica back into solution.

This application is a divisional of U.S. patent application Ser. No.15/467,973, filed Mar. 23, 2017, which claims priority to ProvisionalPatent Application No. 62/315,559, filed Mar. 30, 2016; the disclosuresof which are expressly incorporated herein, in their entirety, byreference. This application also incorporates by reference, in itsentirety, U.S. patent application Ser. No. 15/467,939, filed Mar. 23,2017 and entitled “Colloidal Silica Growth Inhibitor and AssociatedMethod and System,” by Rotondaro et al., which claims priority toProvisional Patent Application No. 62/315,632, filed Mar. 30, 2016.

BACKGROUND

The present disclosure relates to the processing of silicon wafers inphosphoric acid solutions. In particular, it provides a novel method toprevent the growth of colloidal silica deposits on the wafer surfaceduring processing in phosphoric acid.

Phosphoric acid has been used in the semiconductor industry to removesilicon nitride films with high selectivity to silicon dioxide and puresilicon. In 1967, Gelder and Hauser published an article where theyproposed the use of boiling phosphoric acid to remove silicon nitridefilms. They used the boiling point of the acid at a given dilution tocontrol the acid temperature and its concentration in the process tank.This process has been widely adopted in the semiconductor industry andit is used in production today.

A typical application of the boiling phosphoric acid process in thesemiconductor industry is the removal of silicon nitride films. In suchapplications, the silicon nitride film typically sits on a silicondioxide film and it is surrounded by deposited silicon dioxide. Theprocess for the removal of the silicon nitride film begins by a carefulremoval of any remaining silicon dioxide that might rests on top of thesilicon nitride film by a diluted hydrofluoric acid (HF) treatment. Thedilution of the HF is tailored to remove any remaining silicon dioxidefrom the surface of the silicon nitride without significantly removingthe deposited silicon dioxide that was placed between the siliconnitride regions. After the silicon nitride deglaze step in HF, a highselectivity etch is performed in boiling phosphoric acid to remove thesilicon nitride film without removing the deposited silicon dioxide filmthat exists between the silicon nitride films, stopping on the silicondioxide film that lays under the silicon nitride film.

It has been found that during processing of certain semiconductorstructures the adverse growth of colloidal silica deposits on theexposed silicon dioxide regions may hinder silicon nitride removaland/or other subsequent process steps. There is a need for a method toremove the colloidal silica deposition during the treatment of surfacesin phosphoric acid.

SUMMARY

Described herein is an innovative method to remove the growth ofcolloidal silica deposits on surfaces treated in phosphoric acid. Thedisclosed techniques include the use of a high selectivity overetch stepin the phosphoric acid process to remove the deposited colloidal silica.

In one embodiment, the disclosed techniques include the use ofphosphoric acid to remove silicon nitride from structures having siliconnitride formed in narrow gap or trench structures having high aspectratios. The arrangement of such structures is particularly conducive tothe formation of a detrimental amount of colloidal silica deposits on asurface of the narrow gap or trench through a hydrolysis reaction. Whenthe silicon nitride etching is completed, instead of removing thestructure from the phosphoric acid solution, it is left in the solutionwhere an additional chemical reaction now takes place. The hydrolysisreaction which formed the colloidal silica deposits is reversible, andwith the now lower concentration of silica in the nearby phosphoric aciddue to the depletion of the silicon nitride, the equilibrium drives thereaction in the reverse direction, dissolving the deposited silica backinto solution.

In another embodiment, a method for etching features formed on asubstrate is provided. The method may comprise providing a substratehaving a high aspect ratio structure comprised of a first set of exposedfeatures comprising silicon nitride, and a second set of exposedfeatures comprising silicon or silicon oxide. The method furthercomprises loading the substrate into a wet chemical processing system.The method further comprises two etch steps: performing a first etchstep comprised of exposing the substrate to a first wet etch chemicalcomposition to remove at least part of the silicon nitride from the highaspect ratio structure, the exposing the substrate to a first wet etchchemical composition forming silicon containing deposits on the siliconor silicon oxide of the high aspect ratio structure; and performing asecond etch step comprised of exposing the substrate to a second wetetch chemical composition to remove at least some of the siliconcontaining deposits formed on the silicon or silicon oxide during thefirst etch step.

In yet another embodiment, a wet chemical processing system is provided.The wet chemical processing system may comprise a chamber configured toreceive a substrate and expose the substrate to a wet etch chemicalcomposition, a chemical supply system coupled to the chamber, thechemical supply system supplying the wet etch chemical composition tothe chamber, and a controller configured to control components of thewet chemical processing system to execute the techniques describedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present inventions and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features. It is to be noted, however, that theaccompanying drawings illustrate only exemplary embodiments of thedisclosed concepts and are therefore not to be considered limiting ofthe scope, for the disclosed concepts may admit to other equallyeffective embodiments.

FIGS. 1A-1E illustrate cross-section views of an exemplary semiconductorstructure having silicon nitride to be removed and the formation andremoval of silica deposits on the semiconductor structure.

FIG. 2 illustrates an exemplary Si concentration verse time curveillustrating the changing Si concentration during the time period ofsilicon nitride etching and during the time period of silica depositremoval.

FIG. 3 illustrates an exemplary process flow for one embodiment of thesilica deposit removal techniques disclosed herein.

FIG. 4 illustrates a wet chemical processing system which may be used toperform the techniques described herein.

DETAILED DESCRIPTION

It has been found that during phosphoric acid etching of silicon nitrideof certain semiconductor structures the adverse growth of colloidalsilica depositing on exposed silicon dioxide regions may hinder siliconnitride removal during processing. More specifically, silicon nitrideformed in structures having narrow gaps, narrow trenches, and/or highaspect ratios are particularly problematic. Such structures may be foundin a wide variety of semiconductor structures such as logic devices,interconnect structures, Fin Field Effect Transistors (FinFET), 3Dsemiconductor structures, flash memory devices such as Not AND (NAND)logic type memory devices, etc.

As described herein, removal of the silicon containing deposits, such ascolloidal silica deposited on surfaces treated in phosphoric acid isprovided. The disclosed techniques include the use of a high selectivityoveretch step in the phosphoric acid process to remove the depositedcolloidal silica. In one embodiment, the disclosed techniques includethe use of phosphoric acid to remove silicon nitride from structureshaving silicon nitride formed in narrow gap or trench structures havinghigh aspect ratios. As discussed, the arrangement of such structures isparticularly conducive to the formation of a detrimental amount ofcolloidal silica deposits on a surface of the narrow gap or trenchthrough a hydrolysis reaction. When the silicon nitride etching iscompleted, instead of removing the structure from the phosphoric acidsolution, it is left in the solution where an additional chemicalreaction now takes place. The hydrolysis reaction which formed thecolloidal silica deposits is reversible, and with the now lowerconcentration of silica in the nearby phosphoric acid due to thedepletion of the silicon nitride, the equilibrium drives the reaction inthe reverse direction, dissolving the deposited silica back intosolution.

In a phosphoric acid silicon nitride etch, the high selectivity betweenthe silicon nitride and silicon dioxide etch rates is modulated, atleast in part, by the silicon content in the phosphoric acid solution.The higher the silicon content the lower the silicon dioxide etch rate.Since the silicon nitride is little affected by the silicon content, theselectivity between silicon nitride and silicon dioxide etch increasesas the silicon content of the solution increases. However, the increasein the silicon content of the solution results in the growth ofcolloidal silica deposits on the exposed silicon dioxide regions duringprocess. The growth of colloidal silica deposits has a negative impactfor the etch of complex structures, as the colloidal silica deposits canprevent the solution flow into high aspect ratio trenches, significantlyslowing the nitride etch process. Furthermore, the growth of colloidalsilica deposits may have adverse effects on subsequent processing steps.

In one embodiment, the overetch process is done as follows: thestructure containing silicon dioxide and silicon nitride is immersed ina bath of phosphoric acid, generally at 70 wt % to 95 wt % phosphoricacid (remainder water), more preferably at 85 wt % phosphoric acid, at atemperature of 140° C. to 170° C., and more preferably 160° C. In thephosphoric acid, the silicon nitride is etched away according to thefollowing chemical reaction:

SiN(s)+H3PO4+H2O→Si(OH)4+NH4H2PO4

This reaction creates silica (Si(OH)4) in the acid solution, but due tothe high concentration of silica and the low pH, the silica can reactwith itself in a hydrolysis reaction to polymerize and form colloidalsilica:

—Si—OH+HO—Si—→—Si—O—Si—+H2O

This colloidal silica is able to form colloidal particles in the acidsolution or directly on the silicon dioxide surface. Colloidal particlesthat form in the solution also can aggregate on the silicon dioxidesurface. This results in the formation of a silica gel that covers thesilicon dioxide surface and continues to grow so long as there is alocal high concentration of silica due to the nearby etching of siliconnitride.

When the silicon nitride is completely etched away, instead of removingthe structure from the phosphoric acid solution, it is left in thesolution where an additional chemical reaction now takes place. Thepreviously shown hydrolysis reaction is reversible, and with the nowlower concentration of silica in the nearby phosphoric acid due to thedepletion of the silicon nitride, the equilibrium drives the reaction inthe reverse direction, dissolving the deposited silica back intosolution.

Although the deposited silica is silicon dioxide (SiO2) which isidentical to the original silicon dioxide structure that it is formingon, the differences in the oxide may exist. The main difference may bethat the original silicon dioxide is a dense crystalline silicon dioxidewhile the deposited silica is very loose and amorphous, causing them tohave very different etch rates. The selectivity of the silicon nitrideversus silicon dioxide etch rates may be carefully controlled so thatthe deposited silica film can be etched away but there is no significantetching of the original silicon dioxide itself.

Once the deposited silica film is dissolved back into the acid solution,then the structure may be removed from the acid bath for subsequentprocessing steps. The addition of a controlled overetch step is thusprovided to remove the deposited colloidal silica from surface treatedin phosphoric acid. Instead of removing the unwanted silica film using adifferent chemical process, the structures stay in the same acid bath toremove the deposits in the same bath due to the reversing of thechemical reaction that caused the deposition in the first place. In suchan approach, a first etch step comprised of exposing the substrate to afirst wet etch chemical composition to remove at least part of thesilicon nitride from the high aspect ratio structure is used. Theexposing the substrate to a first wet etch chemical composition formssilica deposits on the silicon or silicon oxide of the high aspect ratiostructure. Then a second etch step exposing the substrate to a secondwet etch chemical composition to remove at least some of the silicadeposits formed on the silicon or silicon oxide during the first etchstep is performed. Both the first wet etch chemical composition and thesecond wet etch chemical composition may comprise phosphoric acid, infact being the same phosphoric acid bath. However, other techniques mayutilize the concepts described herein wherein the first and second wetetch chemical compositions are differing.

As described above, the techniques disclosed herein may be applicable toa wide variety of semiconductor structures such as logic devices,interconnect structures, FinFETs, 3D semiconductor structures, NANDflash memory devices, etc. An exemplary structure is shown in FIG. 1A.Such a structure may exist in a 3D NAND device or any of many othersemiconductor devices. It will be recognized that the narrow gap, highaspect ratio structure of FIG. 1A is merely exemplary and those skilledin the art will understand that the techniques described herein may beuseful for the processing of many other structures in which the adverseformation of colloidal silica deposits occurs. Thus the narrow gap, highaspect ratio structure shown in FIG. 1A is merely exemplary of any of awide variety of narrow and/or high aspect ratio structures. For example,the techniques described herein are relevant to high aspect ratio trenchstructures.

As shown in FIG. 1A, an exemplary semiconductor structure 100, which maybe for example a portion of a 3D NAND structure, is shown having siliconnitride formed in narrow, high aspect ratio, gaps. As shown narrow, highaspect ratio, gaps are formed between silicon dioxide (SiO2) layers 102.SiO2 layers 102 may alternatively be Si layers or other oxide layers.Silicon nitride (SiN) layers 104 are formed in the gaps between the SiO2layers. Etching of the SiN layers may then occur by exposing thesemiconductor structure 100 to hot phosphoric acid solution, such as forexample be immersion in a hot phosphoric bath or exposing the structureto a spray of hot phosphoric acid. Due to the etching of the SiN, thereis a high concentration of colloidal silica (Si(OH)4) 110 dispersed inthe phosphoric acid solution near the surface of the SiO2 as shown inFIG. 1B. As shown in FIG. 1B, as the SiN etching occurs, the Si(OH)4concentration increases and colloidal silica 110 deposits to form silicadeposits 112 on the SiO2 surfaces as shown in FIG. 1B. FIG. 1Cillustrates the structure 100 when the SiN layer 104 is completelyremoved. As can be seen in the figure, silica deposits 112 are provideon the SiO2 layers 102. It is noted that in FIG. 1C, relatively uniformsilica deposits are provided. However, in practice, the deposits maybuild up over time at the entrances to narrow gaps or trenches. Thus, asshown in FIG. 1D, the deposits may actually “pinch off” the gaps ortrenches, significantly impacting the effectiveness of the ability ofthe phosphoric acid to etch the remaining silicon nitride. Thus, asshown in FIG. 1D silica deposits 116 may completely close off the gapregion 118 formed between SiO2 layers 102, preventing the complete etchof SiN layers 104.

When the etching of the SiN in the first step of etching ends, theincrease in the concentration of colloidal Si(OH)4 (silica) in thesolution decreases and the silica deposition ends. As described herein,the semiconductor structure 100 continues to be exposed to thephosphoric acid so as to move into second stage of etching. In thissecond etching step, the deposited silica is etched by the phosphoricacid. As described above, due to the nature of the silica deposits, thedeposited silica etches preferentially to the SiO2 layers 102. Thus, asdescribed above, the deposited silica can react in a hydrolysis reactionto polymerize and form colloidal silica that dissolves back into theacid solution. Thus, as shown in FIG. 1E, the continued exposure of thedeposited silica to the phosphoric acid solution dissociates anddissolves the deposited SiO2 so that the deposited silica (silica 112and 116) is removed. Once the deposited silica is dissolved back intothe acid solution, the semiconductor structure may be removed from theacid solution for subsequent processing steps.

The process described above is shown in an exemplary Si concentrationverse time chart of FIG. 2. As shown in FIG. 2, the Si concentration isprovided for a silicon nitride etching process of a structure such asshown in FIG. 1A immersed in a bath of phosphoric acid, generally at 85wt % phosphoric acid (remainder water) at 160° C. As shown in FIG. 2,the bulk silicon concentration (ppm) curve 205 generally increases overtime. This reflects the bulk concentration of silicon generally in thephosphoric acid solution. More particularly, the silicon concentrationincrease over time period 210 which generally corresponds to the periodduring which SiN is being etched. The time period 215 corresponds to theperiod in which the SiN etching no longer occurs. During time period215, the silica deposits are being removed from the semiconductorstructure such as shown in FIG. 1E. Silicon concentration curve 207illustrates the local silicon concentration in the vicinity of the highaspect ratio structure (as opposed to the bulk concentration in thesolution generally as shown by curve 205). As shown, locally inside thehigh aspect ratio structure the silicon concentration rises faster dueto the local silica flux from the silicon nitride etch. As shown, thelocal silicon concentration will ultimately reach a higher value thanthe bulk silicon concentration in the phosphoric acid solution. However,once the silicon nitride etch is finished, the local siliconconcentration in the high aspect ratio structure region shown in curve207 starts to decline and eventually will reach the bulk siliconconcentration value over time as shown in the figure.

As mentioned above, the higher the silicon content is, the lower thesilicon dioxide etch rate that generally results. Further, the siliconnitride etch rate is little affected by the silicon content. Thus, thesilicon content changes shown in FIG. 2 will result in a selectivitychange over time for the etch selectivity between silicon nitride andsilicon dioxide etch. Thus, as shown by curve 207 in FIG. 2, the localetch selectivity increases as the silicon content of the solutionincreases to a maximum at approximately the end of time range 210 atwhich point a maximum silicon nitride to silicon oxide etch selectivitywill be obtained. As the local concentration drops in time range 215,the local selectivity between silicon nitride and silicon oxide willcorrespondingly drop from the maximum selectivity. Thus, there isprovided a technique in which a first and second etch step is provided.The first etch step may correspond to the time range 210 and the secondetch step may correspond to the time range 215. The first etch step hasa silicon nitride to silicon oxide maximum etch selectivity that isgreater than the silicon nitride to silicon oxide etch during at leastportions of the second etch step. In one embodiment, the localized etchselectivity for silicon nitride to silicon dioxide etch rates during thefirst etch step may be greater than 100:1, in some cases greater than300:1 and in some cases greater than 500:1. The localized etchselectivity for silicon nitride to silicon dioxide etch rates during thesecond etch step will be equal or less than the first etch step withselectivity decreasing over time to 10% lower than the first etch stepmaximum selectivity in some embodiments, 30% lower than the first etchstep maximum selectivity in some embodiments and even 50% lower than thefirst etch step maximum selectivity in some embodiments.

The various chemical constituents in the acid solution may be monitoredso as to monitor the progress of the reaction equations described above.Thus, monitoring the concentration of at least one chemical constituentof the first wet etch chemical composition (the composition whilesilicon nitride is being etched) using a chemical monitoring systemincluded in a wet chemical processing system may be performed. Further,monitoring the concentration of at least one chemical constituent of thesecond wet etch chemical composition (the composition while silicadeposits are being removed) using a chemical monitoring system includedin a wet chemical processing system may be performed. For example, themonitoring of the first and second etch steps may be a monitoring of thesilicon concentration shown in FIG. 2. As mentioned, though, monitoringof other chemical constituents may be performed.

It will be recognized that the amount of additional time that thesemiconductor structure is exposed to the phosphoric acid solution afterthe removal of the SiN will vary depending upon numerous processvariables, the structure being etched, the amount of exposed siliconnitride removed, the amount of silica deposits and the various etchprocess variables (acid concentrations, silicon concentrations,temperature, etc.). Thus, it will be recognized that the times shown inFIG. 2 are merely exemplary. In one embodiment, the additional etchingtime for removal of the silica deposits may be in the range of anapproximately 5%-40% overetch time (as a function of the silicon nitrideetch time), and more preferable in the range of a 15%-30% overetch time.

As described in relation to FIGS. 1B-1D, the techniques described hereinare particularly useful for high aspect ratio structures. Thus, layeredstructures having high aspect ratios such as shown in FIGS. 1B-1D, highaspect trench structures, high aspect interconnect structures, etc.would benefit from these techniques. Such high aspect ratio structureswill result in higher concentrations of colloidal silica near thesurface of the structure as the colloidal silica will not diffuse asquickly into the bulk of the acid solution. Thus, the mechanism mayoccur such as shown n FIG. 1D in which the buildup of deposited silicamay substantially narrow the desired gap spacing. As described hereinhigh aspect ratios are considered to be aspect ratios of at least 4:1 orgreater.

The present disclosure provides techniques for performing a siliconnitride etch in a wet chemical processing system. FIG. 3 provides anillustrative process flow 300 for such techniques. It will be recognizedthat the techniques described herein may be advantageously utilized inother process flows. As shown in FIG. 3, step 302 includes providing asubstrate having a high aspect ratio structure comprised of a first setof exposed features comprising silicon nitride, and a second set ofexposed features comprising silicon or silicon oxide. Step 304 includesloading the substrate into a wet chemical processing system. Step 306includes performing a first etch step comprised of exposing thesubstrate to a first wet etch chemical composition to remove at leastpart of the silicon nitride from the high aspect ratio structure. Step308 describes that the exposing the substrate to a first wet etchchemical composition forms silica deposits on the silicon or siliconoxide of the high aspect ratio structure. Step 310 includes performing asecond etch step comprised of exposing the substrate to a second wetetch chemical composition to remove at least some of the silica depositsformed on the silicon or silicon oxide during the first etch step.

As discussed above, in step 306 and step 310 the first wet etch chemicalcomposition and/or the second wet etch chemical composition maycomprises phosphoric acid. Thus, the first and second wet etch chemicalcomposition may be the same or different. In one embodiment, the two wetetch chemical compositions are the same, the second wet etch chemicalcomposition being part of a silicon nitride overetch step.

Various process parameters, singularly or in combination may be selectedwherein at least one parameter of the first etch step or the second etchstep is selected such that substantially all of the silicon containingdeposits formed on the exposed features comprising silicon or silicondioxide during the first etch step are removed during the second etchstep. The at least one parameter may be selected from the groupcomprised of duration of the first etch step, duration of the secondetch step, concentration of at least one chemical constituent of thefirst wet etch chemical composition, concentration of at least onechemical constituent of the second wet etch chemical composition,temperature of the first wet etch chemical composition, temperature ofthe second wet etch chemical composition, flow rate of the first wetetch chemical composition, and flow rate of the second wet etch chemicalcomposition. In this manner, the process conditions and variables may beselected to provide the desired effect of removal of all of the siliconcontaining deposits.

As mentioned above, the techniques provided herein may be utilized inany of a wide range of silicon nitride etching equipment andchemistries. Thus, the techniques to described herein may be utilized inmulti-wafer batch silicon nitride etch systems or single wafer siliconnitride systems. For example, the systems described in more detail inconcurrently filed U.S. patent application Ser. No. 15/467,939, entitledColloidal Silica Growth Inhibitor and Associated Method and System, byRotondaro et al., the disclosure of which is expressly incorporatedherein in its entirety by reference, may be utilized. However, it willbe recognized that other systems may also advantageously utilize thetechniques provided herein. Further, the techniques described hereinalso are not limited to phosphoric acid chemistries but are alsoapplicable to other etch chemistries. In addition, even for the use of aphosphoric acid chemistry, the techniques are not limited to thephosphoric acid with water chemistries and process variables describedherein. For example, a phosphoric acid solution that also includes theuse of additives, inhibitors and/or sulfuric acid and may include arange of temperature, concentration and other process variables may beutilized, as described in more detail in concurrently filed U.S. patentapplication Ser. No. 15/467,939, entitled Colloidal Silica GrowthInhibitor and Associated Method and System, by Rotondaro et al., thedisclosure of which is expressly incorporated herein in its entirety byreference. Further, though described herein in a system in which thesilicon nitride etch and the deposited silica removal steps areperformed in the same phosphoric acid solution, it will be recognizedthat the two steps may be accomplished with differing wet etchcompositions. Thus the silicon nitride etch composition and the silicaremoval etch composition may be the same wet etch composition or may bediffering wet etch compositions. It will be therefore recognized thatthe techniques described herein for the removal of deposited silica in asilicon nitride etch system are applicable to a wide range of etchingsystems and etching chemistries.

In one embodiment, the methods described herein may be accomplished in awet chemical processing system. The wet chemical processing system maycomprise a chamber configured to receive a substrate and expose thesubstrate to a wet etch chemical composition and a chemical supplysystem coupled to the chamber for supplying the wet etch chemicalcomposition to the chamber. The wet chemical processing system mayfurther comprise a controller configured to control components of thewet chemical processing system to execute the methods disclosed herein.The wet chemical processing system may also comprise a chemicalmonitoring system for monitoring the concentration of at least onechemical constituent of the wet etch chemical composition. The chemicalconstituent monitored, in one embodiment may be selected from the groupconsisted of silicon, silica, and a Si-containing compound. The wetchemical processing system may be a single substrate processing systemor configured to process a plurality of substrates at the same time.

FIG. 4 illustrates one exemplary wet chemical processing system 400. Itwill be recognized that the techniques described herein may be utilizedwith a wide variety of other wet chemical processing systems. A chamber402 is provided. The chamber is configured to receive a substrate andexpose the substrate to a wet etch chemical composition. The chamber maybe a single wafer chamber or may be a chamber for process multiplewafers such as a batch wet etch tank. A chemical supply system 403 maycomprise a wet chemical source 404 and recirculation lines 406 and 408which provide wet chemicals to and from the chamber. A controller 410 iscoupled to the chamber and the chemical supply system to control and/orreceive feedback from the various components of the wet chemicalprocessing system 400 via signal lines 412 and 414. In one exemplaryembodiment, the controller 410 may be a processor, microcontroller, orprogrammable logic device in combination with other circuitry such asmemory, I/O ports, etc. In one embodiment, the processor,microcontroller or programmable logic device may be configured toexecute instructions or configuration files to perform the functionsdescribed herein.

Further modifications and alternative embodiments of the inventions willbe apparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illustrative onlyand is for the purpose of teaching those skilled in the art the mannerof carrying out the inventions. It is to be understood that the formsand method of the inventions herein shown and described are to be takenas presently preferred embodiments. Equivalent techniques may besubstituted for those illustrated and describe herein and certainfeatures of the inventions may be utilized independently of the use ofother features, all as would be apparent to one skilled in the art afterhaving the benefit of this description of the inventions.

What is claimed is:
 1. A wet chemical processing system for etchingfeatures formed on a substrate having a high aspect ratio structureincluding a first set of exposed features including silicon nitride, anda second set of exposed features including silicon or silicon oxide, thewet chemical processing system comprising: a chamber configured toreceive the substrate and expose the substrate to a wet etch solution ofphosphoric acid; a chemical supply system coupled to the chamber, thechemical supply system supplying the wet etch solution of phosphoricacid to the chamber; and a controller operably coupled to the chemicalsupply system and configured to cause the wet chemical processing systemto: perform a first etch step that exposes the substrate to a wet etchsolution of phosphoric acid to selectively remove at least part of thesilicon nitride from the high aspect ratio structure, wherein ahydrolysis reaction creates a silica reaction product in the first wetetch solution from the silicon nitride, and at least a portion of thesilica reaction product deposits on the silicon or silicon oxide of thehigh aspect ratio structure; and perform a second etch step that furtherexposes the substrate to the wet etch solution of phosphoric acid toremove at least some of the deposited silica reaction product from thesilicon or silicon oxide by dissolving the deposited silica reactionproduct back into the wet etch solution of phosphoric acid, wherein thefirst etch step is performed until a concentration of silica reactionproduct in the wet etch solution of phosphoric acid decreases due todepletion of the silicon nitride available for the hydrolysis reaction,and the second etch step is performed continuously after performing thefirst etch step by continuously exposing the substrate to the wet etchsolution of phosphoric acid and uses the decreased concentration ofsilica reaction product in the wet etch solution to reverse thehydrolysis reaction and dissolve the deposited silica reaction productback into the wet etch solution of phosphoric acid.
 2. The system ofclaim 1, further comprising a chemical monitoring system that monitors aconcentration of at least one chemical constituent of the wet etchsolution of phosphoric acid.
 3. The system of claim 2, wherein the atleast one chemical constituent is selected from a group consisted ofsilicon, silica, and a silicon containing compound.
 4. The system ofclaim 1, wherein the wet chemical processing system is a singlesubstrate processing system.
 5. The system of claim 1, wherein the wetchemical processing system is configured to process a plurality ofsubstrates at the same time.
 6. The system of claim 1, wherein thesecond etch step is a silicon nitride overetch step.
 7. The system ofclaim 1, wherein at least one parameter of the first etch step or thesecond etch step is selected such that substantially all of thedeposited silica reaction product formed on exposed features comprisingsilicon or silicon dioxide during the first etch step is removed duringthe second etch step.
 8. The system of claim 7, wherein the at least oneparameter is selected from a group consisting of duration of the firstetch step, duration of the second etch step, concentration of at leastone chemical constituent of the wet etch solution during the first etchstep, concentration of at least one chemical constituent of the wet etchsolution during the second etch step, temperature of the wet etchsolution during the first etch step, temperature of the wet etchsolution during the second etch step, flow rate of the wet etch solutionduring the first etch step, and flow rate of the wet etch solutionduring the second etch step.
 9. The system of claim 1, wherein a maximumsilicon nitride to silicon oxide etch selectivity during the first etchstep is higher than the silicon nitride to silicon oxide etchselectivity during at least a portion of the second etch step.
 10. A wetchemical processing system for etching features formed on a substratehaving a high aspect ratio structure with an aspect ratio of at least4:1 including a first set of exposed features including silicon nitride,and a second set of exposed features including silicon or silicon oxide,the wet chemical processing system comprising: a chamber configured toreceive the substrate and expose the substrate to a wet etch solution ofphosphoric acid; a chemical supply system coupled to the chamber, thechemical supply system supplying the wet etch solution of phosphoricacid to the chamber; and a controller operably coupled to the chemicalsupply system and configured to cause the wet chemical processing systemto: expose the substrate to a wet etch solution of phosphoric acid togenerate a chemical etching reaction for a first time duration toselectively remove at least part of the silicon nitride from the highaspect ratio structure, wherein during the first time duration aconcentration of silica in the wet etch solution from the chemicaletching reaction initially increases and a hydrolysis reaction occursthat forms a colloidal silica reaction product that deposits on thesilicon or silicon oxide of the high aspect ratio structure, and whereinthe first time duration ends when the concentration of silica decreasesin the wet etch solution due to depletion of silicon nitride availablefor the chemical etching reaction; after the first time duration,continue to expose the substrate to the wet etch solution of phosphoricacid for a second time duration to reverse the hydrolysis reactionthereby dissolving the deposited silica reaction product back into thewet etch solution of phosphoric acid to preferentially removesubstantially all of the deposited colloidal silica reaction productfrom the silicon or silicon oxide without etching the silicon or siliconoxide; and monitor a concentration of silica, silicon or asilicon-containing compound in the wet etch solution using a chemicalmonitoring system included in the wet chemical processing system todetermine completion of the first and second time durations.
 11. Thesystem of claim 10, wherein the wet chemical processing system is asingle substrate processing system.
 12. The system of claim 10, whereinthe wet chemical processing system is configured to process a pluralityof substrates at the same time.
 13. The system of claim 10 wherein amaximum silicon nitride to silicon oxide etch selectivity during thefirst time duration is higher than the silicon nitride to silicon oxideetch selectivity during the second time duration.
 14. The method ofclaim 10 wherein the wet etch solution of phosphoric acid is at atemperature of 140° C. to 170° C. for the first time and second timeduration.
 15. The system of claim 10 wherein the wet etch solution ofphosphoric acid is 70 wt % to 95 wt % phosphoric acid.
 16. The system ofclaim 10 wherein the wet etch solution of phosphoric acid is at atemperature of 140° C. to 170° C. for the first time and second timeduration and the wet etch solution of phosphoric acid is 70 wt % to 95wt % phosphoric acid.